The effect of human recombinant erythropoietin (r-EPO) on primary hemostasia was studied in 9 patients undergoing hemodialysis. The analysis was performed before and after the hematocrit reached a value of 30%. The most important complications observed during the study period were a death for acute myocardial infarction in a patient with previous severe ischemic cardiopathy and a thrombosis of the venous line. The bleeding time shortened in four patients although the mean value did not change significantly. Platelet count showed a non significant increase. There was a significant improvement in in vitro platelet aggregability with ADP (p less than 0.05), arachidonic acid (p less than 0.05), adrenaline (p less than 0.05), and ristocetin (p less than 0.05) as well as in the parameters that quantify the interaction between platelets and subendothelium in in vitro experiments using perfused models (p less than 0.05). There were no significant changes in coagulation and fibrinolysis tests. The treatment with r-EPO improved primary hemostasia in uremic patients. This beneficial effect was due to the increased hematocrit and to the improvement of platelet function induced by r-EPO.